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Marketable Securities - Available-for-sale Securities by Major Security Type (Details) - USD ($)
$ in Thousands
Jun. 30, 2021
Dec. 31, 2020
Marketable Securities [Line Items]    
Amortized cost $ 1,976 $ 2,848
Gross unrealized holding gains 21 31
Gross unrealized holding losses
Estimated fair value 1,997 2,879
Corporate bonds and notes [Member]    
Marketable Securities [Line Items]    
Amortized cost 1,007 1,312
Gross unrealized holding gains 19 26
Gross unrealized holding losses
Estimated fair value 1,026 1,338
Municipal bonds [Member]    
Marketable Securities [Line Items]    
Amortized cost 969 1,536
Gross unrealized holding gains 2 5
Gross unrealized holding losses
Estimated fair value $ 971 $ 1,541